1. Field of the Invention
The present invention is directed to a method for displaying acoustic signal transit times that occur within a body part suitable for obtaining information therefrom by acoustic irradiation, of the type wherein ultrasound transmission signals are transmitted into the body part along scan lines by an ultrasound transducer arrangement, the ultrasound signals that have passed through the body part are acquired as transmitted acoustic signals with a further ultrasound transducer arrangement which is disposed opposite the ultrasound transducer arrangement, and wherein in acoustical signal times occurring along the scan lines are calculated from respective transmission points in time of the ultrasound signals and corresponding reception points of time of the transmitted sound signals.
The invention is likewise directed to an apparatus for implementing the method of the type having an ultrasound transducer arrangement for transmitting ultrasound transmission signals along scan lines into the body part, a further ultrasound transducer arrangement arranged opposite the transmitting ultrasound transducer arrangement for acquiring ultrasound signals that have passed through the body part as transmitted sound signals, a processing unit connected to the two ultrasound transducer arrangements for identifying acoustic signal transit times from the transmitted sound signals, and a display for visual presenting the results.
2. Description of the Prior Art and Related Subject Matter
In ultrasound diagnostics, body regions or body parts are scanned with ultrasound pulses for producing anatomical tomograms. The image data are thereby acquired from echo signals or reflection signals that are triggered at boundaries of adjoining regions respectively having different acoustic characteristic impedances inside the body. The characteristic impedance is the product of the density of, and the speed of sound in the tissue types. The locus coordinates for the source of the echo signals in the tomogram on the display is obtained from the chronological spacing of the echoes from the transmitted acoustic pulse and from the propagation direction of the acoustic beam, i.e. from the position of the scan line in the sectioned plane. The transmitted acoustic beam is focused as sharply as circumstances permit. The chronological spacing is calculated in ranges or distances using an average speed of sound, amounting to 1540 m/s for soft biological tissue. The boundary surfaces of different acoustic impedances that, for example, represent organ boundaries, vessel boundaries or fine internal structures, are reproduced with relatively good geometrical precision in the tomograms obtained in this way, one form thereof being referred to as B-images.
No acoustic propagation properties that are present inside the body part itself, or inside the examination region, can be derived from this type of image presentation. Such acoustic propagation properties, for example, are the speed of sound or the acoustic absorption.
Knowledge of the acoustic propagation properties, however, is of significance for clinical diagnosis. In many instances, for example, in solid tumors the speed of sound is higher than in healthy tissue. These slight differences in the speed of sound cannot be recognized in the normal image presentation. When, for example, different acoustic propagation speeds are found within a scanned section plane, this normally leads to a slight geometrical distortion in the ultrasound tomogram that cannot be recognized. No conclusion about the speed of sound along the individual scan lines can be derived from the tomogram.
There are, however, known image presentation methods that, for example, are disclosed in European Application 0 097 917 or U.S. Pat. No. 4,075,883 that can directly display the geometrical distribution of such parameters. The acoustic signal transit time and/or the absorption are thereby measured in an acoustic irradiation method. After acoustically irradiating the examination region from a suitable number of directions, the local distribution of the speed of sound or of the absorption is calculated from the measured values with methods similar to those known from computed tomography. These methods are extremely time-consuming and employ complicated algorithms. They have not been able to find acceptance in the practical clinical routine essentially for two reasons.
First, there are only few body parts, such as mammaries and gonads that are suitable for acoustic irradiation. Second, measuring errors as a consequence of refractions occur in ultrasound computed tomography that appear as image unsharpnesses in the computer tomogram.
An acoustic irradiation method of the type initially cited is known from the article by Y. Hayakawa, entitled "Mass Screening of Breast Cancer by Ultrasound Transmission Technique-Theoretical Considerations", Proceedings of the Fifth Symposium on Ultrasound Electronics, Tokyo 1984, which appeared in the Japanese Journal of Applied Physics, Vol. 24 (1985), Suppl. 24-1, pp. 82-83. A method suitable for mass screening of the breast is proposed therein wherein average speed of sound is measured and after subtraction of, for example, the speed of sound of water, the result is displayed. In practice, however, this method has not proven satisfactory.
German patent application P 43 09 596.8 (published after the priority date of the present application) discloses that regions having different speeds of sound can be recognized in the ultrasound image when a boundary surface having a precisely known geometrical decision is located behind the structure to be diagnosed. Changes in transit time can then be recognized at this "reference surface" in the form of deviations from the "normal" geometrical appearance of this boundary surface in the displayed image. The reference surface can be produced, for example, by attaching a planar reflector plate following (in the direction of sound propagation) the body part which is acoustically irradiated. A possible disadvantage of this technique is that only echo signals can be evaluated which have passed through the examination region in the forward and return directions which may result in the generation of stronger reflection echoes than arise from the reference surface. Moreover, this reference echo can be completely absent from or imperceptible in the image as a consequence of acoustic absorption, particularly because of the double acoustic propagation path and the acoustic scatter of the reflected part in the examination region.